ASK-IT aims to harness the combined power of potable devices, wireless communicationprotocols, bespoke service development, and adaptive interface design, to deliver on-demandservices for mobility impaired users. A key service to be provided by ASK-IT is localisation.The context for the research presented in this paper is a user wishing to navigate within abuilding, or building complex. This may be on a room-by-room basis, or localising to aspecific coordinate relative to a building plan–

an x,y position. Wireless sensor networks usesmart dust, or MOTEs to form ad-hoc networks. Using trilateration, a position can bedetermined for an unknown MOTE located within a fixed network ofMOTEs. A reasonableaccuracy of between 0.5-1.2 meters has been achieved using the techniques presented here.ASK-IT International Conference–

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1. Introduction

The growth of mobile computing has been unprecedented. Mobile phones and personalcomputing assistants (PDAs) are now widespread;

their use being distributed across mostpopulation demographics, with this trend set to continue. The combination of portability andthe increasing coverage of wireless communications mean that Internet and phone services arebeing delivered to a wider and more technicallysavvy

are being used in connection with the latest technologicaladvances and the buying public are quick to test these concepts in the real world. Combiningportability, network access, the hardware capabilities of devices and network accessiblyinformation, provides a dynamic, flexible and extensible platform for research developmentand evaluation. The ASK-IT project[7]

is harnessing this platform to aid a user base with aspecific set of needs.

The mobility-impaired (MI) user covers a broad range of impairments. While avoidingsingling out any one set of users as in need of special attention, the reality is that certain userprofiles are not as yet included in the accessible information age in which we now live;yetthis population. One of the key aims of the ASK-IT project is to bridge the gap-

bringing theclear benefits of on demand information service to all users, regardless of their personalcircumstances. It’s a challenge, but a laudable one.

ASK-IT harnesses the combined power of device, communication, bespoke servicedevelopment, and adaptive interface design, to deliver on-demand services. These services arefiltered and delivered in a way considered most accessible to the user concerned. For example,the visually impaired user might desire an audible interface, with voice-activated commands.Cross-modal communications where sound, vision and perhaps haptics may be used incombination, to both capture users’ commands and convey system events. To this end, ASK-IT is concerned with helping users to access and interpret information across a wide range ofeveryday activities: tourism and travel, navigation, domotics, and access to information–

bridging the gap between the virtual space and the real world ofeveryday peoples’ lives. Assuch, the project has identified key, high level services, which when combined can satisfy aset of clearly defined use-cases. By combining these use cases, usage scenarios have beendeveloped before being evaluated. Users willaccess services through commonly availabledevices such as phones and PDAs, as well as a specifically deigned device being developed aspart of the project.

A key service to be provided by ASK-IT is localisation. The context for the research presentedinthis paper is a user wishing to navigate within a building, or building complex. This may beon a room-by-room basis, or localising to a specific coordinate relative to a building plan–

anx,y position. The goal of assisted route guidance in predicated on knowing where one isbefore being able to navigate to where one wants to be. For the ASK-IT project a localisationservice is being developed which will calculate and deliver a position (encoded as a JSR179[8]

address) to the

requester. This position can then be displayed on a map, or plan andpresented to the user in a form suited to their impairment.

Wireless Sensor Networks use Smart Dust, or MOTES to form ad-hoc networks. An ad-hocnetwork is one that is formed from a set

of communication nodes, which come together in anad-hoc way. They allow for very flexible and dynamic network topologies to be formed inASK-IT International Conference–

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real-time. Nodes are allowed to join and leave the network at any time. The network is brokenif any two nodes, or groups of nodes, do not have a connecting node. A connection is madewhen the signal pattern (the area of active communication) of one MOTE intersects with thatof a neighbouring MOTE. Network topology is therefore an important factor as is the distancebetween MOTES. Given this infrastructure, groups of MOTES within a building for examplecan form an ad-hoc network, which can then be exploited for various applications. An activeMOTES network provides a communications platform through which data can be propagatedand received between on-board pluggable sensors, with applications in many areas. Forexample, environmental monitoring and control is possible through such an infrastructure. Inthe case of localisation, it is the communication characteristics of the MOTES that provide aunique opportunity for localisation. Using well-understood received signal strengthcharacteristics it is possible to determine the distance between two points on a network giventhe signal strength. If this calculation is repeated across a number of MOTES for a given pointthen a position can be determined. By using a fixed network within the building and have aroaming,mobile

MOTE, which is able to move through the network while still remainingconnected, it is possible to determine the position of the mobile MOTE relative to the fixednetwork. If the fixed MOTES are mapped onto a building plan, then a position relative to thebuilding can be determined.

This paper presents work on the effectiveness of localisation techniques for indoorlocalisation using the MicaZ MOTE and sensor board. The purpose is to determine accuracyand highlight any issues that may need to be overcome in order to use a MOTE basedlocalisation service for ASK-IT. The technique used is trilateration. This technique is forMOTES in a two-dimensional plane[6]. ASK-IT requires localisation within a three-dimensional network, so by using a correcting algorithm to map from 3D to 2D a position canbe found at any point and height. This paper is structured as follows: Section 2 discussesMOTES and their use in localisation and also reviews existing work in the area; Section 3describes scenarios from which a set of requirements for internal localisation using MOTEScan be determined; Section 4 describes localisation techniques used in this work; Section 5presents the evaluation of the results relative to the requirements; Section 6 concludes thepaper.

2.

Wireless Sensor Networks

Wireless sensor networks is a name given to two or more devices that communicate with eachother and also incorporate an ability to access, or sense, their immediate environment.

Because of the computational power neededto run such protocols, most devices used in awireless sensor network contain on-board processors. The generic term for one of thesedevices is a mote. It is widely accepted that motes are the modern day precursors to nanometre(10x10-6m) sized smartdustdevices.

Many manufacturers of motes incorporate a modular design consisting of a main board withthe processor, memory, wireless device, aerial and batteries. A connector on this mainboardASK-IT International Conference–

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allow the addition of daughterboards that can contain sensors such as temperature, light,sound, magnetism etc. There are also application specific sensors such as GPS.

This project will be using the Crossbow Technologies Mica2 motes, transmitting at 906MHz.Before designing a wireless sensor network, the characteristics of the motes needs be takeninto consideration. These include inter-mote distance and mote height from the ground. In[1][2]

it is suggested that a mote height of around 100 cm is optimal and

a viablecommunication distance of around 50-55 meters.

There are several methods of determining the position of a single mote in a network: range-based or range-free. Broadly speaking, a range-based approach determines the range of onemote to anotherby measuring the distance between the two motes using the received signal;this could be by measuring the signal propagation time or comparing the received signalstrength against a theoretical or empirical model to determine distance. Although using thereceived signal strength indicator is deemed a bad estimator of link quality, work has beencarried out to prove that new systems based on IEEE 802.15.4 standard can provide a reliableindicator of link quality and hence provide justifiable distances[3].

Range free methods generally include anchor motes that have fixed locations, the locations ofmobile motes in the network can be determined by the calculation of the relative distances toseveral anchor motes–

a process generally referred to as triangulation[5]. Other techniquesinclude ring overlapping which is reported to achieve a more accurate location estimate thanthe triangulation method[5].

3.

Localisation System Scenarios

To test and evaluate the use of wireless sensor networks (WSN) for effective indoorlocalisation for MI users, a series of usage scenarios have been devised. Each will evaluate theeffectiveness and suitability of WSN for a given set of requirements. The requirements at thisstage are only concerned with measurable parameters sufficient to fulfil the scenario aims.

Zones

Localisation is often associated with a particular point (x,y,z coordinate), however, this levelof detail is not always a requirement. The zones scenario described here is one such case inpoint and has a particular relevance to ASK-IT use-cases.

The ASK-IT use-case 4.2 illustrates a system interaction where a user requires help via humaninvolvement. For example, this might bethe case where a MI user may have fallen, or they’relost in unfamiliar surroundings. In-order to provide assistance the helper must be able tolocate the subject. An additional scenario, might be a building that must be evacuated and aspart of that evacuation able helpers are dispatched to locate and assist any mobility-impairedpersons still in the building. A possible system is illustrated inFigure1. The setup of thenetwork involves having a numbered MOTE placed in each room,

and any connectingcorridors-

each MOTE can be queried in turn from the server using its unique identifier. Theplacement of the MOTES must be such that they form an ad-hoc network. The mobile MOTEis associated with a particular user. An additional advantage of this system is that bothattendance and location can be determined through the one system; if the user is present andwhere the user is located.In this case the location is a room.

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Figure1. Zones Localisation Scenario

Following the call to evacuate, most likely through sounding of the fire alarm, the scenarioruns as follows:Figure1.1 the system requests the location of all mobile MOTEs;Figure1.2Each fixed MOTE is asked to report the signal strengths of all MOTEs visable within itssignal footprint;Figure1.3 the server determines form the signal strengths received where themobile MOTES is located-

as the fixed MOTE positions are known, then the mobile positionwill be the room where the fixed MOTE reported the highest signal strength when connectingto the mobile MOTE.Figure1.4 the system informs the able bodies helper where the MI usercan be found by matching the known location of the fixed MOTE to the localted mobileMOTE.Figure1.5 is an optional part to this scenarios involves the server sendinginformation/instructions to the MI user via a WiFi network to their PDA display.

In this

scenario the server requests the location of the mobile MOTE. Given the need to locateMI users then this is appropriate. The same setup is able to test the case where the MI userrequests their location. They must send a localisation request to the server via the MOTESnetwork. On receiving this request the localisation is done and the result sent.

Open Access

Room, or zone level granularity is not always sufficient. Consider a visually impaired userwanting to navigate through an unfamiliar space, for example an office or canteen area, or alarge shopping center. For this they will need localisation based on an accurate position. Thisscenario illustrates a plausible use-case for determining a coordinate position within an openspace.

Figure2. Open Area Localisation Scenario

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In this scenario the MI user initiates the localisation request.

Figure

2.1 the MI user’s device requests a localisation coordinate from the system passing theidentifier for the mobile MOTE they have; this is sent via the WiFi network to the server.

Figure

2.2 the server requests the signal strength data from all MOTES in the network.

Figure

2.3 each MOTE reports the signal strength between itself and any other visibleMOTE.Figure2.4 the server filters the results using only those signals between thestatic MOTEs and the mobile MOTE in question. Using these data and a 3D to 2DASK-IT International Conference–

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mapping algorithm determines the x,y position of the mobile MOTE.

Figure2.5 the position is mapped relative to the room layout and sent to the MI user’s PDAvia the WiFi network.

Requirements for scenarios

Examining the scenarios above a set of high-level requirements for the system to support themcan be derived.Table1

shows a list of requirements with a description of each. Theevaluation discussion will refer back to this list to demonstrate the validity of the approachtake.

Requirement

Discussion

Network

Network connectivity is a key component of the system. The nature of ah-hocnetworks are such that provided nodes are within range of their neighbours,then the network is connected. The network required consists of a fixed set ofMOTEs and mobile MOTEs, which roam while remaining connected. Thetopology of the fixed network needs to be such that all MOTEs are connected,with a link to the server from one MOTE. Although we can locate

within a 3Dnetwork, the fixed motes must be in the same plane.

Connectivity

The topology of the fixed network needs to be understood and mapped to thephysical space.

SignalStrength

The signal strength is required in order to determine the position of the mobileMOTE. To understand how this parameter performs a calibration exercise iscarried out.

Mapping

A position of the mobile MOTE relative to the fixed network must bedetermined. In-order to find this position relative to the building or room, theposition must be mapped to building plans and room layouts.

Interface

At this stage a simple layered interface showing a map layout and therequested position is sufficient to demonstrate results.

Accuracy

A calibration and determination of the accuracy of the MOTEs for localisationunder a set of conditions typical of those found in the scenarios.

Table1

Requirements

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The above requirements are suitable for a system that enacts all stages of the scenariospresented. This pape

ris focusses in evaluating only the use of signal strength for finding aposition of a given set of MOTEs.

4. Localisation using MOTES

For this work trilateration was used to localise a mobile MOTE within a static fixed network.Trilateration uses simplegeometry to determine the locale of a variable point using itsdistance from a set of known fixed points. Using this method a minimum of 3 fixed points isrequired. MOTEs must be in a single plane.

“Standing at B, you want to know your location relative

to the reference points P1,P2, and P3. Measuring r1 narrows your position down to a circle. Next, measuringr2 narrows it down to two points, A and B. A third measurement, r3, gives yourcoordinates at B. A fourth measurement could also be made to reduce

error.”[6]

Figure3

Trilateration technique for localisation on a 2D plane

Calibration Testing

Many factors affect the outcome of localisation using motes-

interference caused by furnitureand fitting, reflections from surfaces and walls can cause fluctuations or non-linearcharacteristics. In the test area an attempt to minimise these errors by mote placement, andclearing of furniture was done, however, these conditions are normal in a real world situation.ASK-IT is aiming to localise users in many different contexts, so although aclean

test areawould give the best calibration and testing result, a more realistic setup would give realisticresults and help to answer the question as to whether motes

can be used for the ASK-ITindoor localisation service. The test area is Space 2 in Culture Lab on Newcastle University’scity campus, seeFigure4.

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Figure4

Space 2 Culture Lab, Newcastle University

The first stage, and in order to determine effective localisation, calibration of the equipment isrequired. The method to be used requires an accurate distance between the known fixed motesand the mobile mote.Figure5

below showsa plot of signal strength over distance. This wasdetermined by first marking a line with ½ meter gradations on the floor of the test area to adistance of 5 meters. A receiving mote was placed at the 0m mark. Then, one by one witheach mote in the test set, a mote was place at ½ meter intervals starting from 1m up to 5m. Ateach point the received signal strength was measured. This measurement was repeated 30times and an average taken for each mote at each gradation. As can be seen inFigure5

a veryconsistent signal to distance ration is seen for all the test motes. From 0-3.5 meters areasonably linear relationship is shown. Beyond 3.5 meters the signal becomes less linear. Toachieve a good result it was therefore decided to limit the range of the motes to 3.5 meters soas to give the most accurate locale for the mobile mote.

Trilateration is performed using the signal to distance ratio to determine the distance betweena transmitting and receiving mote on a single 2D plane. This will determine the position of anunknown mote within a network of fixed motes, where their positions are known. This willnot, however, be the case in a real-world scenario such as an ASK-IT user wishing to navigateinside a shopping centre of hospital complex. In this case the fixed network is likely to belocated on the ceiling or high enough on the wall infrastructure to be out of harms way. Theuser will carry the mobile mote. With the fixed motes at ceiling height and the mobile mote atapproximately 1 meter height, a single plane is not possible-

the motes are now arranged in a3-dimentional network. To compensate for this a projection from the 3D to 2D is needed.

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Figure6

3D to 2D projection

Figure6

above illustrates how the MOTE localisation compensates for the 3-dimentionalaspect of the tests. The red and green dots represent mobile motes, each positioned at adifferent height.

5. Evaluation

The evaluation was split into two parts. The first looked at localising in a 2D plane. For this aset of four fixed motes where positioned to form a network at approximately 1 meter abovethe floor level. The positions of the motes were recorded. A fifth mote was then place in thenetwork. It’s position was also recorded relative to the previous four so that a comparisonbetween the actual position and the calculated position could be made. A series of eight testswhere then carried out and the results recorder. Each test was repeated multiple times to showgroupings of positions. For each test the fifth mote was rotated through 45 degrees. This wasto see what affect this would have on the resulting position calculated. In the real world a userwill be actively moving and turning so this was considered important to examine.

Figure7

Results for 2D network with mote rotation

The results inFigure7

show a wide range of positions. Those for 45, 90 and 135 degrees arethe closest to the expected with only ½ meter error from the actual position. The full range ofresults shows errors of up to 6 meters from the expected. It is difficult to determine why thisASK-IT International Conference–

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is. Perhaps through signal reflection, or signal pattern as the groupings for each angle suggestthat this characteristic does have an impact.

The tests were repeated for a 3D network. The fixed motes were positioned on the lightingtracks with the locations projected on to the floor plan so the mobile motes location could bemeasured relative to the fixed motes.

Figure8

Results for 3D network with mote rotation

The results inFigure8

show a better position,with a good grouping average positions andmost results within 1 meter of the expected. The overall spread of results was better for the 3Dnetwork. It is not clear as to why this is the case.

6. Conclusion

This paper has presented work on evaluating the suitability of MicaZ wireless motes forindoor localisation. The technique used enabled a mobile mote to be localised within a staticnetwork of motes of known positions. Both 2D and 3D networks where examined. The resultsfor both networks produced a position with an accuracy of ½ meter for certain mote rotationangles relative to the fixed network. An encouraging result was the 3D network as thisshowed greater consitency across the full range of angles with an accuracy to within 1.2m forall rotation angles.